Traditionally a cam lobe grind has been split into several separate increments typically five increments. Thus if it was necessary to remove a total of 2 mm depth of stock on the radius, the depth of material removed during each of the increments typically would be 0.75 mm in the first two increments, 0.4 m in the third increments, 0.08 mm in the fourth, and 0.02 mm in the last increment.
Usually the process would culminate in a spark-out turn with no feed applied so that during the spark-out process, any load stored in the wheel and component was removed and an acceptable finish and form is achieved on the component.
Sometimes additional rough and finish increments were employed, thereby increasing the number of increments.
During grinding, the component is rotated about an axis and if the component is to be cylindrical, the grinding wheel is advanced and held at a constant position relative to that axis for each of the increments so that a cylindrical component results. The workpiece is rotated via the headstock and the rotational speed of the workpiece (often referred to as the headstock velocity), can be of the order of 100 rpm where the component which is being ground is cylindrical. Where a non-cylindrical component is involved and the wheel has to advance and retract during each rotation of the workpiece, so as to grind the non-circular profile, the headstock velocity has been rather less than that used when grinding cylindrical components. Thus 20 to 60 rpm has been typical of the headstock velocity when grinding non-cylindrical portions of cams.
Generally it has been perceived that any reduction in headstock velocity increases the grinding time, and because of commercial considerations, any such increase is unattractive.
The problem is particularly noticeable when re-entrant cams are to be ground in this way. In the re-entrant region, the contact length between the wheel and the workpiece increases possibly tenfold (especially in the case of a wheel having a radius the same, or just less than, the desired concavity), relative to the contact length between the wheel and the workpiece around the cam nose and base circle. A typical velocity profile when grinding a re-entrant cam with a shallow re-entrancy will have been 60 rpm around the nose of the cam, 40 rpm along the flanks of the cam containing the re-entrant regions, and 100 rpm around the base circle of the cam. The headstock would be accelerated or decelerated between these constant speeds within the dynamic capabilities of the machine (c & x axes), and usually constant acceleration/deceleration has been employed.
The power demand on the spindle motor driving the grinding wheel is dictated in part by the material removal rates i.e. the amount of material the wheel has to remove per unit time. The increased contact length in the re-entrant regions has tended to increase this and very high peak power requirements have been noted during the grinding of the concave regions of the flanks of re-entrant cams.
For any given motor, the peak power is determined by the manufacturer, and this has limited the cycle time for grinding particularly re-entrant cams, since it is important not to make demands on the motor greater than the peak power demand capability designed into the motor by the manufacturer.
Hitherto a reduction in cycle time has been achieved by increasing the workspeed used for each component revolution. This has resulted in chatter and burn marks, bumps and hollows in the finished surface of the cam which are unacceptable for camshafts to be used in modern high performance engines, where precision and accuracy is essential to achieve predicted combustion performance and engine efficiency.
The innovations described herein have a number of different objectives.
The first objective is to reduce the time to precision grind components such as cams especially re-entrant cams.
Another objective is to improve the surface finish of such ground components.
Another objective is to produce an acceptable surface finish with larger intervals between dressings.
Another objective is to equalise the wheel wear around the circumference of the grinding wheel.
Another objective is to improve the accessibility of coolant to the work region particularly when grinding re-entrant cams.
Another objective is to provide a design of grinding machine, which is capable of rough grinding and finish grinding a precision component such as a camshaft, in which the cam flanks have concave regions.
These and other objectives will be evident from the following description.